Cleanup imports

Author: taiki-e

src/libserialize/hex.rs

@@ -145,7 +145,7 @@ impl FromHex for str {
 #[cfg(test)]
 mod tests {
     extern crate test;
-    use self::test::Bencher;
+    use test::Bencher;
     use crate::hex::{FromHex, ToHex};
 
     #[test]

src/libserialize/json.rs

@@ -199,7 +199,6 @@ use std::ops::Index;
 use std::str::FromStr;
 use std::string;
 use std::{char, f64, fmt, str};
-use std;
 
 use crate::Encodable;
 

src/libserialize/opaque.rs

@@ -1,6 +1,6 @@
 use crate::leb128::{self, read_signed_leb128, write_signed_leb128};
-use std::borrow::Cow;
 use crate::serialize;
+use std::borrow::Cow;
 
 // -----------------------------------------------------------------------------
 // Encoder